Effect of droplets generation in the boiling limit conditions: Heat pipe visualization

Abstract

Heat pipe devices, for their typical working mechanisms, are particularly suitable for zero gravity applications, and have also been considered for applications in space satellites with nuclear propulsors, thanks to the absence of mobile systems for the coolant fluid circulation. The present work reports the results of the experimental tests carried out on a conventional cylindrical heat pipe designed to investigate on possible thermal-hydraulic performance improvements and to evidence the importance of the droplets generation that determines the boiling limit and consequently the heat transport capability. Some of the tests showed that the droplets generation, due to the nucleate boiling regime, can cause a liquid recirculation and a consequent reduction of liquid evaporation with a heat transport capability lower (about 250%) than that estimated from capillary (wicking) limit conditions. For this reason, generally, to predict more realistically the capability for given wick size, it is not sufficient to know the tilt angle, the total length and the diameter of the tube. Some short grooves (cuts of 15 mm in length) made in the screens at the end of the evaporating zone demonstrate their efficiency to increase the heat flux values more than 2 or 3 W/cm 2. The same heat flux increasing, concerning the wick-wall contact improvement, is obtained thickening the coils of the spring used to tight the wick against the internal tube wall.